The present invention is directed to a vehicle skid control system. Specifically, the subject invention is directed to a skid control system for controlling the brake pressure independently of an input metered pressure when the input metered pressure is in excess of a predetermined control pressure, which predetermined threshold pressure is generated by the control valve in response to a control signal generated by the skid control circuitry of the system.
A vehicle skid control system generally includes a servovalve which operates in two modes: a metered pressure mode in which the input metered pressure is not above a predetermined control pressure; and a skid control mode in which the input metered pressure exceeds the predetermined control pressure. In the metered pressure mode, the metered pressure is applied directly to a brake port. An input control signal to the skid control valve determines the control pressure and in the event the input metered pressure exceeds the control pressure, the skid control valve controls the brake port pressure in response to the input control signal. Since in the skid control mode an input control signal controls the brake port pressure, the gain of a skid control valve can be defined as the change in brake pressure which results from a given change in control signal level. Generally, in skid control operation brake pressure follows the control pressure and a skid control valve can also be evaluated in terms of control pressure generated in response to a current control input.
Prior art skid control systems include fixed gain as well as variable gain type valves. However, these prior art systems had the disadvantage of having a different pressure control-signal level relationship for a given metered pressure input. That is, the brake pressure output in the skid control mode of operation would depend upon the control signal level as well as the input metered pressure. As a result, skid control effectiveness is diminished because of the effect variation of input metered pressure has upon the brake pressure applied in response to a particular control input signal.
Some of the disadvantages of such prior art systems include the tendecy to release excessive brake pressure and great sensitivity of the brake pressure applied to control signal changes at low brake pressures. As noted above, sensitivity to input metered pressure variation is also an important disadvantage.
These prior art systems include skid control valves which generate a control pressure in response to the control signal and have the metered input pressure as the source of supply pressure for generating the control pressure. Therefore, in addition to the metered input pressure dependence discussed above, brake pressure reapplication is delayed because of flow drop across the pressure metering valve during pressure recovery.
It is therefore an object of the subject invention to provide an improved skid control system.
A further object of the present invention is to provide an improved skid control system having a control valve which operates independently of input metered during skid control and thereby provides a single gain response characteristic in the skid control mode.
Still another object of this invention is to provide a skid control system having a valve which generates a predetermined control pressure for comparison with an input metered pressure.
A further object of the present invention is to provide an improved skid control system in which valve brake port pressure is controlled independently of input metered pressure whenever the metered pressure is in excess of a predetermined control pressure.
Another object of the present invention is to provide a skid control valve having rapid pressure reapplication which is insensitive to the input metered pressure.